(1) Field of the Invention
The present invention relates to a heterojunction bipolar transistor, and particularly to an InGaP/GaAs heterojunction bipolar transistor.
(2) Description of the Related Art
Heterojunction bipolar transistors (HBT) using semiconductor with large bandgap for emitter have been in practical use as high-frequency analogue elements for mobile phones and the like. In particular, an InGaP/GaAs HBT using InGaP as an emitter has small temperature dependency of current gain (HFE) since discontinuity of valence band (ΔEv) is large, and is expected to be widely used as a highly reliable device.
A device structure of a general InGaP/GaAs HBT is described hereafter with reference to a diagram (for example, see Japanese Laid-Open Patent Application No. 2000-260783).
FIG. 1 is a diagram showing a structure of an npn-type InGaP/GaAs HBT.
As shown in FIG. 1, in the InGaP/GaAs HBT, an n+-type GaAs sub-collector layer 202 which is formed by doping an n-type impurity at a high concentration is stacked on a semi-insulating GaAs substrate 201. On the sub-collector layer 202, a GaAs collector layer 203 that is either non-doped or of n-type with low impurity concentration, p-type GaAs base layer 207 with a high-impurity concentration, and an n-type InGaP emitter layer 208 are sequentially stacked. These layers form a convex portion formed only on a range of the sub-collector layer 202. Furthermore, on the emitter layer 208, an n-type GaAs emitter cap layer 209 and a low-resistance n-type InGaAs emitter contact layer 210 are sequentially stacked. The emitter cap layer 209 and the emitter contact layer 210 form the second convex portion formed only on the emitter layer 208.
Here, on the emitter contact layer 210, an emitter electrode 251 made of, for example, Ti/Pt/Au and the like, is formed. On the emitter layer 208, a base electrode 252 made of multi-layer metal and the like including Pt is formed by heat diffusion on the emitter layer 208 that is exposed in the periphery of the emitter cap layer 209 so as to contact the base layer 207. On the sub-collector layer 202, a collector electrode 253 made of AuGe/Ni/Au and the like is formed. Furthermore, in order to electrically separate the HBT from other HBT, an element isolation region 254 ranging from the sub-collector layer 202 to the substrate 201 is formed by ion implantation and inactivated heat treatment in an element peripheral region of the substrate 201 and the sub-collector layer 202.